Episodic memory is particularly susceptible to age-related decline. Research in animal models and cross-sectional studies in humans provide evidence that engaging in aerobic exercise may mitigate the effects of aging on memory. Yet, despite abundant evidence from animal literature, many aerobic exercise interventions in humans have failed to demonstrate improvements in learning and memory (Erickson et al., 2011a; Tamura et al., 2014; Ngandu et al., 2015). Further, we know little about how aerobic exercise affects brain function and whether functional changes are behaviorally relevant. In the present study, we investigated the effect of a 6-month aerobic exercise intervention on episodic memory retrieval using fMRI in healthy older adults. Specifically, 137 older adults (60 - 80 years of age) were randomized across four treatment groups: walking, walking plus nutrition, dance, and stretching and toning (active control). Although we found that the walking groups increased cardiorespiratory fitness (CRF) relative to the dance and control groups, there were no group differences in episodic memory. Consistent with previous studies, we found no association between change in CRF and change in memory performance. However, we did find that change in CRF, across intervention groups, was positively associated with change in hippocampal activation during recognition 'hits'. Furthermore, increased hippocampal activation was associated with improvements in memory performance. Thus, these results demonstrate a mechanism by which increases in CRF may indirectly relate to memory performance improvements in the absence of a significant behavioral effect.